Millions of elderly Americans lack economic access to essential medications due to insufficient insurance coverage. The Medicare Modernization Act of 2003 (MMA) will allow Medicare beneficiaries to purchase a prescription drug benefit (Part D), beginning in January 2006. Additional subsidies will be available to those with low-incomes or very high drug costs. Important questions exist about the impact of Part D on medication use. With NIA funding, we developed new measures to track behaviors collectively known as cost-related underuse of medicines (CRUM). In 2004, one set of CRUM measures was incorporated into the Medicare Current Beneficiary Survey (MCBS;N~15,000 per year), the principal national survey for designing and evaluating health policies for Medicare enrollees. The principal goal of this investigation is to measure changes in drug coverage, use, spending, and CRUM among elderly Medicare beneficiaries before and after implementation of the MMA (2005-2007), with a particular focus on poor and chronically ill beneficiaries who will qualify for substantially subsidized coverage and near-poor beneficiaries who will not. Our study will use 6 years of MCBS panel data prior to the MMA and 2 years afterwards (2000-2007). We will stratify the sample into three mutually-exclusive income groups which, when we account for pre-MMA dual eligibility status, have different potential susceptibility to MMA benefits. We will also focus on hypertension and diabetes, two chronic medical conditions that are highly prevalent, identifiable in this dataset, and for which effective medicines are available and clinically beneficial. The specific aims will be: (1) describe the prevalence, trend, and year-to-year individual changes in insurance coverage for prescription drugs, generosity of coverage, total drug utilization, use of highly effective drugs for diabetes and hypertension, and drug expenditures during the six-year period prior to the MMA (2000 to 2005);(2) using new measures of CRUM, describe pre-MMA (2004-2005) patterns of CRUM by income and chronic illness group, including associations between changes in the prevalence and generosity of coverage and changes in CRUM;and (3) in cohort- and individual specific analyses, evaluate the impact of the MMA (2005-2007) and associated changes in coverage generosity on total drug utilization and expenditures, CRUM and use of highly effective medicines among patients with diabetes and hypertension. We hypothesize that low income enrollees without Medicaid coverage who enroll in Part D will have increased access to effective drugs and reduced CRUM. The MMA represents an unprecedented re-organization of prescription drug coverage for American seniors. The proposed longitudinal research will measure the impact of the MMA on cost-related barriers in access to appropriate drug therapy among poor, near-poor, and chronically ill seniors.